NEC in the Facility: Load Calculations, Part 3

Oct. 9, 2012
When calculating branch circuit loads, what's the maximum load you’re allowed? 

When calculating branch circuit loads, what's the maximum load you’re allowed? The limit is this: The total calculated load on a given branch circuits can't exceed the rating of that circuit [220.18]. When calculating, don't forget to multiply continuous loads by 125%.

But suppose you're designing a fabrication shop. The tenant owns a single arc welder and needs to use it all over the shop. So you run a single branch circuit with 10 receptacles on it. Since only one receptacle will be used at any time, what's the calculated load? Answer: It's the load presented by that single arc welder. This assumes there are controls (administrative or otherwise) in place to ensure there won’t be a second arc welder (perhaps a rented one) also in use.

The principle is "load diversity," which the NEC mentions in several places [e.g., 310.15(B)(3)] but doesn't define. Account for load diversity so that you don't grossly oversize the conductors.

A common example for residential applications is the fact that you aren't going to run air-conditioning and electric heat at the same time. Therefore, you don't need to size the service to accommodate both loads; you need to accommodate only the larger of the two. Only when two loads are non-coincidental (or, ideally, mutually exclusive) should you consider them diverse.

Voice your opinion!

To join the conversation, and become an exclusive member of EC&M, create an account today!

Sponsored Recommendations

Electrical Conduit Comparison Chart

CHAMPION FIBERGLASS electrical conduit is a lightweight, durable option that provides lasting savings when compared to other materials. Compare electrical conduit types including...

Fiberglass Electrical Conduit Chemical Resistance Chart

This information is provided solely as a guide since it is impossible to anticipate all individual site conditions. For specific applications which are not covered in this guide...

Considerations for Direct Burial Conduit

Installation type plays a key role in the type of conduit selected for electrical systems in industrial construction projects. Above ground, below ground, direct buried, encased...

How to Calculate Labor Costs

Most important to accurately estimating labor costs is knowing the approximate hours required for project completion. Learn how to calculate electrical labor cost.